1. Field of the Invention
The invention relates to an active matrix substrate partially constituting a liquid crystal display device, and a method of fabricating the same.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a conventional active matrix substrate 100 partially constituting a liquid crystal display device.
The active matrix substrate 100 is comprised of a glass substrate 101, a thin chromium (Cr) film 102 formed as a gate electrode partially on the glass substrate 101, a silicon nitride film 103 formed as an electrically insulating film, covering the thin chromium film 102 and the glass substrate 101 therewith, an active layer 104 formed on the silicon nitride film 103, n+ doped amorphous silicon film 105 formed partially on the active layer 104, a thin chromium (Cr) film 106 formed as a barrier film on the n+ doped amorphous silicon film 105, and an indium tin oxide (ITO) film 107 which will make a pixel electrode and which makes contact with the thin chromium film 106 and covers the silicon nitride film 103 therewith.
The active matrix substrate 100 is fabricated as follows.
First, the thin chromium film 102 which will define a gate electrode is formed on the glass substrate 101 by sputtering. Then, the thin chromium film 102 is patterned into a gate electrode.
Then, the silicon nitride film 103, the active layer 104 and the n+ amorphous silicon film 105 are successively formed on the glass substrate 101 by plasma-enhance chemical vapor deposition (PECVD) at 300 degrees centigrade.
Then, a data wiring layer comprised of the active layer 104 and then n+ doped amorphous silicon film 105 is patterned into an island by photolithography and dry etching.
Then, the thin chromium film 106 is formed on the n+ doped amorphous silicon film 105 by sputtering. The thin chromium film 106 acts as a barrier layer between the data wiring layer and the ITO film 107.
Then, the thin chromium film 106 and the n+ doped amorphous silicon film 105 are patterned.
Then, the ITO film 107 which will define a pixel electrode is formed by sputtering, and then, is patterned.
Thus, the active matrix substrate 100 including a thin film transistor having an amorphous silicon film, as a switching device, is fabricated through the above-mentioned steps.
Since glass has a high specific gravity, the active matrix substrate 100 including the glass substrate 101 is relatively heavy.
In particular, since glass is readily broken, the glass substrate 101 has to be formed to have a great thickness, resulting that the active matrix substrate 100 is unavoidably heavy.
These days, a liquid crystal display device is required to be light and thin, and hence, an active matrix substrate which is a part of a liquid crystal display device has to be fabricated lighter and thinner.
However, for the reasons mentioned above, there is limitation in fabricating a liquid crystal display device including a glass substrate, lighter and thinner.
Consequently, in order to fabricate a liquid crystal display device lighter and thinner, many attempts have been made to use a resin substrate in place of a glass substrate, because a resin substrate is lighter than a glass substrate and can be fabricated thinner than a glass substrate.
For instance, Japanese Unexamined Patent Publication No. 11-103064 (A) has suggested an active matrix substrate including a thin film transistor (TFT) as a switching device which thin film transistor is comprised of a thin polysilicon film formed on a resin substrate.
A thin film transistor includes a gate insulating film as an indispensable part. A gate insulating film is formed generally by plasma-enhanced chemical vapor deposition (PECVD) or sputtering.
A resin substrate generally has about 200 degrees centigrade as a maximum resistance to heat. The inventors had conducted various experiments, and found out that a gate insulating film formed by PECVD or sputtering at 200 degrees centigrade or lower, which is a maximum resistance of a resin substrate to heat, would have a low density and cause much current leakage, resulting in that the gate insulating film was not practicable. Accordingly, even if steps other than a step of forming a gate insulating film were carried out at 200 degrees centigrade or lower, it would be impossible to form a high-quality gate insulating film.
In the above-mentioned experiments, the inventors had also found out that a gate insulating film formed by PECVD or sputtering at 300 degrees centigrade or higher had a high density and had caused only small current leakage, and hence, the gate insulating film was sufficiently practicable.
However, 300 degrees centigrade is over a maximum resistance of a resin substrate to heat. Hence, if PECVD or sputtering were carried out at 300 degrees centigrade or higher for forming a gate insulating film, a resin substrate would be thermally destroyed.
Japanese Unexamined Patent Publication No. 10-173194 (A) has suggested a method of fabricating a semiconductor device, including the steps of forming a first inorganic insulating thin film on a resin substrate or resin film without exposing a surface on which the first inorganic insulating thin film is to be formed, to plasma, forming a second inorganic insulating thin film on the first inorganic insulating thin film with the surface being exposed to plasma, and forming a thin semiconductor film on either the first inorganic insulating thin film or the second inorganic insulating thin film.
Japanese Unexamined Patent Publication No. 11-174424 (A) has suggested a substrate to be used for a liquid crystal display panel which substrate is composed of copolymer polycarbonate resin containing 3,3,5-trimethyl-1,1-di(4-phenol) cyclohexyridene, bisphenol, and bisphenol constituents wherein the bisphenol is contained in the range of 30 to 99 mol %.
Japanese Unexamined Patent Publication No. 7-74374 (A) has suggested a thin film diode including a first electrode layer formed on a substrate, a semiconductor layer formed on the first electrode layer, a buffer layer formed on the semiconductor layer, and a second electrode layer formed on the buffer layer, wherein the semiconductor layer and the buffer layer have almost the same pattern as each other.
The above-mentioned problem remains unsolved even in the above-mentioned Publications.
In view of the above-mentioned problem in the prior active matrix substrates, it is an object of the present invention to provide an active matrix substrate which includes a resin substrate and is capable of avoiding thermal destruction of a resin substrate.
In view of the shortcomings in the above-mentioned conventional active matrix substrates, the inventors paid attention to a diode which is not necessary to include a gate insulating film. That is, the inventors selected a diode as a switching device to be used for an active matrix substrate, in place of a thin film transistor.
In one aspect of the present invention, there is provided an active matrix substrate including (a) a substrate composed of resin, and (b) a polysilicon thin film diode formed on the substrate.
The active matrix substrate in accordance with the present invention is not necessary to include a gate insulating film having low quality and low reliability, unlike a conventional active matrix substrate including a thin film transistor, ensuring enhancement in performances and reliability.
In addition, it is possible to use a resin substrate having a smaller thickness than a glass substrate in the active matrix substrate in accordance with the present invention. Hence, in comparison with an active matrix substrate including a glass substrate, it would be possible to reduce a height of an active matrix substrate, and hence, a height of a liquid crystal display device including the active matrix substrate in accordance with the present invention.
It is preferable that the polysilicon thin film diode is formed as a lateral diode.
If the polysilicon thin film diode were formed as a vertical diode, it would be necessary to carry out film deposition and laser annealing a plurality of times. If an upper film is annealed by radiating laser beams thereto, a profile of an impurity concentration in a lower film might be destroyed. Furthermore, if film deposition and laser annealing were not carried out in vacuum, a natural oxidation film would be formed between layers. Since a lateral diode can be formed without causing such problems as mentioned above, it is preferable that the polysilicon thin film diode is formed as a longitudinal diode.
It is preferable that the lateral diode centrally has a region into which impurity is doped.
The lateral diode may be designed to have a nin structure, a pip structure, an ini structure or an ipi structure.
As an alternative, the lateral diode may be designed to have ni- or pi-Schottky structure.
The polysilicon thin film diode may be comprised of two lateral diodes electrically connected in parallel to each other and arranged in opposite directions.
The substrate may be composed of polyethersulfon, polyimide, polycarbonate or siloxane.
The active matrix substrate may be designed to further include a light-shielding film formed below the polysilicon thin film diode.
The light-shielding film may be comprised of a chromium film.
The active matrix substrate in accordance with the present invention may be applied to a light-transmission type liquid crystal display device, a COT type liquid crystal display device or a light-reflection type liquid crystal display device.
In another aspect of the present invention, there is provided a method of fabricating an active matrix substrate, including the steps of (a) forming an amorphous silicon film on a substrate composed of resin, (b) doping impurity into the amorphous silicon film in a selected region thereof, (c) radiating laser beams to the amorphous silicon film for crystallizing the amorphous silicon film into a polysilicon film, and (d) patterning the polysilicon film into an island to thereby form a parallel-type diode.
There is further provided a method of fabricating an active matrix substrate, including the steps of (a) forming an electrically insulating film on a substrate composed of resin, (b) forming an amorphous silicon film on the electrically insulating film, (c) doping impurity into the amorphous silicon film in a selected region thereof, (d) radiating laser beams to the amorphous silicon film for crystallizing the amorphous silicon film into a polysilicon film, (e) patterning the polysilicon film into an island, (f) forming a metal wiring such that the metal wire makes electrical contact with the island-shaped polysilicon film, (g) forming an interlayer insulating film all over a product resulted from the step (f), (h) forming a contact hole through the interlayer insulating film such that the contact hole reaches the metal wire, and (i) forming a metal film which will define a pixel electrode such that the contact hole is filled with the metal film.
The metal film to be formed in the step (i) may be an electrically conductive transparent film such as an indium tin oxide (ITO) film. The metal film may be annealed.
The method may further include the step of (j) annealing the polysilicon film. The step (j) is to be carried out between the steps (d) and (e).
The method may further include the step of (k) applying hydrogen plasma to the polysilicon film.
The method may further include the step of (l) forming a light-shielding film on the resin substrate.
An active matrix substrate formed by the above-mentioned methods may be applied to a light-transmission type liquid crystal display device or a COT type liquid crystal display device.
There is still further provided a method of fabricating an active matrix substrate, including the steps of (a) forming an electrically insulating film on a substrate composed of resin, (b) forming an amorphous silicon film on the electrically insulating film, (c) doping impurity into the amorphous silicon film in a selected region thereof, (d) radiating laser beams to the amorphous silicon film for crystallizing the amorphous silicon film into a polysilicon film, (e) patterning the polysilicon film into an island, (f) forming a metal wiring such that the metal wire makes electrical contact with the island-shaped polysilicon film, (g) coating a photosensitive film over a product resulted from the step (f), exposing the photosensitive film to a light, and developing the photosensitive film to thereby form base steps in a region in which a pixel is to be formed, (h) forming an interlayer insulating film all over a product resulted from the step (g), (i) forming a contact hole through the interlayer insulating film such that the contact hole reaches the metal wire, and (j) forming a metal film which will define a pixel electrode such that the contact hole is filled with the metal film.
The method may further include the step of (k) annealing the base steps for smoothing the base steps, the step (k) being to be carried out between the steps (g) and (h).
The interlayer insulating film may be formed of the same material as the material of which the base steps are formed, in the step (h).
The method may further include the step of annealing the metal film.
An active matrix substrate formed by the above-mentioned methods may be applied to a light-reflection type liquid crystal display device.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
The active matrix substrate in accordance with the present invention is no longer necessary to include a gate insulating film having low quality and low reliability. Hence, the active matrix substrate in accordance with the present invention could present enhanced reliability in comparison with a conventional active matrix substrate including a thin film transistor.
In the active matrix substrate and the method of fabricating the same both in accordance with the present invention, there is not used a thin film such as an amorphous silicon film to be formed by PECVD which thin film is necessary, when formed, to produce a process temperature equal to or higher than a maximum resistance of a resin substrate to heat. Accordingly, the active matrix substrate and the method of fabricating the same both in accordance with the present invention make it possible to use a resin substrate in place of a glass substrate. The active matrix substrate including a resin substrate can be formed lighter and thinner than an active matrix substrate including a glass substrate. This ensures that a liquid crystal display device including the active matrix substrate can be formed lighter and thinner than a liquid crystal display device including a conventional active matrix substrate having a glass substrate.
The method of fabricating an active matrix substrate in accordance with the present invention makes it possible to reduce the number of photoresist steps in which photolithography and etching are carried out through the use of a photoresist film, in comparison with a conventional method of fabricating an active matrix substrate including a thin film transistor. Specifically, a conventional method of fabricating an active matrix substrate including a thin film transistor was necessary to carry out photoresist steps six or seven times. In contrast, the method of fabricating an active matrix substrate in accordance with the present invention carries out photoresist steps only five times.
In addition, the active matrix substrate in accordance with the present invention includes a resin substrate thinner than a glass substrate. Accordingly, it would be possible to reduce a height of the active matrix substrate in accordance with the present invention in comparison with an active matrix substrate including a glass substrate. Hence, it would be possible to reduce a height of a liquid crystal display device including the active matrix substrate in accordance with the present invention in comparison with a liquid crystal display device including a conventional active matrix substrate including a glass substrate.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.